1. Field of the Invention
The present invention relates to a load-control-type actuator for use in handling equipment for mounting and assembling components and measuring and sorting equipment for measuring electrical characteristics of electronic components.
2. Description of the Related Art
Hitherto, when mounting small-size electronic components on a circuit board, component assembling apparatuses using voice-coil motors have been widely used. The voice-coil motor is excellent in positioning accuracies more than those of mounters using pneumatic power and solenoids and is also excellent in reducing a force applied to components.
FIG. 1 shows an example of a conventional handling apparatus using such a voice-coil motor. A movable part 120 of a Z-axis-drive mechanism is driven by a voice-coil motor in the vertical direction. A nozzle 121 is arranged at lower end of the movable part 120 and a built-in spring 122 is arranged interposing between the movable part 120 and the nozzle 121. In the nozzle 121, a sucking port 121a is formed, which is communicated with a vacuum sucking device (not shown) via a sucking port 120a of the movable part 120a. A workpiece W is thereby held at tip end of the nozzle 121. On the external periphery of the nozzle 121, a flange 121b is unitarily formed, which is downwardly urged by a spring 122 in touch onto a receiving portion 120b formed at the lower end of the movable part 120 and the flange 121b is held thereto.
An initial load is applied to the spring 122 in order to hold the nozzle 121 when the movable part 120 moves vertically as described above. Accordingly, an impact load produced when the nozzle 121 touches the workpiece W or when the nozzle 121 having the workpiece W held thereon touches an object (circuit board, etc.) is certainly lager than the initial load, product defects such as cracks and chips are caused during handling or mounting small-size and thin elements.
Such problems are not limited to handling equipment; in measuring and sorting equipment, for example, the problem also rises when a measuring probe is urged onto an electronic component so as to measure electrical characteristics of the electronic component.
In component-assembling equipment having the sucking nozzle 121 , when air leakage is generated due to non-existence of a sucked component or disagreement in sucking, a sucking force by air and the initial load of the spring 122 are out of balance so that the initial load becomes larger causing damages of workpieces during the handling. Furthermore, since the sucking port 121a is released when a workpiece W is to be picked up by sucking, the initial load does not become large so much; however, when placing the workpiece W on a circuit board, etc., after picking it up, because the sucking port 121a of the nozzle 121 is closed, the nozzle 121 is pulled toward the movable part 120 so that the initial load becomes larger. Therefore, there is a problem that the load applied to the workpiece W during the placing is larger than during the picking.
Accordingly, it is an object of the present invention to provide a load-control-type actuator in which an impact load applied to a workpiece can be reduced so as to prevent damages in the workpiece.
In order to achieve the above-mentioned object, a load-control-type actuator is provided, which comprises a contact part which comes into contact with a workpiece, a movable part for supporting the contact part via an elastic member, and operating means which can control a thrust driving force to the movable part, wherein provided that F1max denotes the maximum impact load at a time of contact between the workpiece and the contact part and F2 denotes a spring reaction force of the elastic member within a response time tk of the actuator, a spring constant kb of the elastic member is set to satisfy the condition: F1maxxe2x89xa6F2.
A component assembling apparatus having a sucking nozzle for picking up and placing components will be described as an example. In the sucking nozzle having a spring built therein, the impact force to a workpiece is broken down into three forces shown in FIG. 2.
A first force is an initial load (A); a second force is an impact load (B) due to rigidity parameter of a colliding object; and a third force is a load (C) due to a reaction spring force of a built-in spring.
From FIG. 2, it is understood that when the initial load (A) included in a conventional apparatus is eliminated, the entire impact load can be reduced.
However, since the initial load (A) is for holding the nozzle to be stable, a spring having high (hard) rigidity has to be built-in in order to have the zero initial load. When the rigidity is too high, however, since the load (C) due to a reaction spring force of a built-in spring shown in FIG. 2 influences thereon significantly, the spring constant thereof needs to be suitably designed.
Therefore, according to the present invention, the spring constant kb of the elastic member is set so that the spring reaction force F2 of the elastic member within the response time tk of the actuator is reduced smaller than the maximum impact load F1max between the workpiece and the contact part.
Thereby, the impact load applied to the workpiece is reduced enabling damages in the workpiece to be reduced small. Moreover, since the contact part can be held to be stable while reducing the initial load to be small, the movable part can be operated in a high speed.
The spring constant kb of the elastic member can be obtained from the equation:
F2=kbexe2x88x92xcex5tV0/q sin(qt),
provided that 0xe2x89xa6txe2x89xa6tk, and the equations:
e=cb/2mv, q=[xcfx892xe2x88x92xcex52]xc2xd, and xcfx892=kb/mv,
wherein mv is the mass of the movable part and cb is the spring viscosity.
The maximum value of the spring constant kb of the elastic member can be thereby calculated.
When the operating means is formed by a voice-coil motor, it is preferable that the response time tk range from 0.001 to 0.1 second.
A voice-coil motor is a type of a linear actuator and produces a thrust driving force in proportion to a current. When a current I is passed through a voice coil from a controller via a current amplifier, a force of F=B I L is produced in the coil by a magnetic flux density B of a magnetic circuit and a length L of a coil conductor. By the force F, the nozzle (movable part) connected to the coil via a connecting mechanism is moved. When a nozzle mechanism including the coil is lowered for handling the workpiece, for example, the nozzle can be precisely positioned (below 0.1 mm) based on a signal of a positional sensor. As described above, the impact load to the workpiece is suppressed by controlling a collision velocity and by controlling a current after collision, a precise static load is applied when approaching the workpiece.
In addition, when using the voice-coil motor, the practical range of the response time tk is approximately from 0.001 to 0.1 second.
Preferably, the operating means sequentially performs positional control so that the movable part is advanced to a position immediately before the contact part touches a workpiece, velocity control so that the movable part is advanced at a constant velocity from the position immediately before the contact to the contact position at which the contact part touches the workpiece, load control so that the movable part is further advanced while controlling the contact pressure between the contact part and the workpiece after the contact part touches the workpiece, and positional control so that the movable part is retracted so as to separate the contact part from the workpiece.
By controlling the operating means sequentially in such a manner, a load applied to the workpiece is reduced, enabling the high-speed operation to be performed with a high degree of accuracy.
The spring constant kb of the elastic member may preferably satisfy the minimum conditional equation:
kb greater than Mn(xcex1+g)/xmax,
wherein mn is the mass of the contact part, xcex1 is the change in velocity of the contact part at a point switching from the positional control to the velocity control, g is the gravitational acceleration, and Xmax is the maximum amplitude of the movement of the contact part at point of switching from the positional control to the velocity control.
That is, immediately before the contact part touches the workpiece, the control is switched from positional to velocity and after the switching, the contact part is slowly brought into contact with the workpiece, so that an impact load applied to the workpiece can be reduced. When the spring constant of the elastic member is too small (spring is too soft), however, the contact part vibrates vertically at point of contact so as to be unstable. Therefore, by satisfying the above-mentioned minimum conditional equation, the vibration of the contact part is suppressed so that the stable contact can be maintained when the contact part is brought into contact with the workpiece.
Preferably, the contact part is suspended from the movable part via the elastic member and the initial load of the contact part to the workpiece is zero.
That is, a comparatively hard spring is adopted as the elastic member which is assembled thereinto with an initial load of zero, so that the movement of the contact part can be stabilized while reducing the impact load.